1. Technical Field
The present invention relates to an optical branching device in which an optical signal consisting of plural wavelengths transmitted through one optical fiber is divided, with respect to wavelength, so as to be transmitted into two optical fibers.
2. Background Art
In the field of optical transmission systems, various kinds of optical functional devices such as optical switches, optical isolators, optical circulators, optical attenuators, and wavelength-selective filters are required. These optical functional devices may be connected with an optical fiber directly or may be connected via an optical member called a “collimator lens”. A structure in which a collimator lens is connected with an optical fiber is called an “optical fiber collimator”. The optical fiber collimator has a function in which a light flux emitted from an end surface of an optical fiber is refracted into a parallel light flux by a collimator lens and is transmitted into an optical member, or in which a parallel light flux transmitted through an optical member is converged by a collimator lens and is transmitted into an optical fiber.
As a combined optical system (hereinafter called an “optical module”) comprising the optical functional device and the optical fiber collimator, an optical branching device is known. The optical branching device has a function in which an optical signal consisting of plural wavelengths transmitted through an optical fiber is divided, with respect to wavelength, and is transmitted into two optical fibers. Therefore, a wavelength-selective filter for dividing an optical signal with respect to wavelength is used in the optical branching device. Optical modules such as optical branching devices are desirably manufactured at low cost. As a technique in which an optical module is provided at low cost, a technique disclosed in Japanese Unexamined Patent Application Publication No. 2004-279708 is known.
FIGS. 7A and 7B show an example of a conventional optical branching device. The optical branching device shown in FIG. 7A comprises a wavelength-selective filter 57 positioned on a V-shaped groove portion of a base plate 58. Both sides of the wavelength-selective filter 57 comprise an optical fiber collimator 61 and an optical fiber collimator 62, respectively.
The optical fiber collimator 61 comprises two optical fibers 52 and 53, a capillary 51 for holding the optical fibers, and a planoconvex lens 50. The capillary 51 comprises through holes at positions symmetric with respect to the optical axis of the planoconvex lens 50, and it holds the optical fibers 52 and 53 in the through holes. The end surfaces of the optical fibers 52 and 53 are bonded to a transmitting plane surface of the planoconvex lens 50.
The optical fiber collimator 62 comprises an optical fiber 55, a capillary 56, and a planoconvex lens 54. The capillary 56 comprises a thorough hole and holds the optical fiber 55 therein. The through hole of the capillary 56 and the through hole of the capillary 51 for holding the optical fiber 53 are symmetric with respect to the wavelength-selective filter 57. The end surface of the optical fiber 55 is bonded to a transmitting plane surface of the planoconvex lens 54.
As shown in FIG. 7B, the wavelength-selective filter 57 and the optical fiber collimators 61 and 62 are held together by using a pressing base plate 59 having the same shape as the base plate 58. Moreover, the entire structure is surrounded by a holding plate 60 made from a thin metal, and the clearance thereof is filled with a setting adhesive so as to form a hermetic structure. According to the structure, light which has a wavelength λ2 and which entered from the optical fiber 52 is reflected at the wavelength-selective filter 57 and enters into the optical fiber 53. On the other hand, light which has a wavelength λ1 and which entered from the optical fiber 52 is transmitted through the wavelength-selective filter 57, and the light is converged at the optical fiber 55 and is output from the optical fiber 55.
For example, techniques relating to the above technique are disclosed in Japanese Unexamined Application Publication Nos. 2005-234441 and 2006-209085. In the technique, only a collimator lens having a lower softening point than that of an optical fiber is softened, and an optical fiber is fusion bonded to the softened collimator lens.
In these conventional techniques, a wavelength-selective filter is required in addition to a collimator lens. The wavelength-selective filter functions as a reflecting mirror with respect to a certain wavelength, and a holding member (jig) is thereby required for accurately adjusting the position and the direction and holding the wavelength-selective filter. Moreover, a complicated operation for the adjustment is required.
When a wavelength-selective filter is required as additional parts and a holding member for adjusting the position and the direction of the wavelength-selective filter is also required, the cost of parts may be increased. Moreover, when adjustments of the position and the direction of a wavelength-selective filter are required for each product, manufacturing cost may be increased. Therefore, the optical branching device in the above conventional techniques would often be expensive.